<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../vue.js"></script>
</head>

<body>
    <div class="app">
        姓：<input type="text" v-model="firstname"><br><br> 名：
        <input type="text" v-model="lastname"><br><br> 全名：
        <!-- 不推荐写法 -->
        <span>{{firstname.slice(0,3)}}{{lastname.slice(0,3)}}</span>
    </div>
    <script>
        const vm = new Vue({
            data() {
                return {
                    firstname: '张',
                    lastname: '三'
                }
            }
        })
        vm.$mount('.app')
    </script>
</body>

</html>